The field of the invention relates generally to high voltage direct current (HVDC) transmission systems and, more particularly, to hybrid HVDC converter systems and a method of operation thereof.
At least some of known electric power generation facilities are physically positioned in a remote geographical region or in an area where physical access is difficult. One example includes power generation facilities geographically located in rugged and/or remote terrain, for example, mountainous hillsides, extended distances from the customers, and off-shore, e.g., off-shore wind turbine installations. More specifically, these wind turbines may be physically nested together in a common geographical region to form a wind turbine farm and are electrically coupled to a common alternating current (AC) collector system. Many of these known wind turbine farms include a separated power conversion assembly, or system, electrically coupled to the AC collector system. Such known separated power conversion assemblies include a first converter station, i.e., a rectifier that converts the AC generated by the power generation facilities to direct current (DC). Such known assemblies also include a second converter station, i.e., an inverter that converts the DC to AC of a predetermined frequency and voltage amplitude. The first converter station is positioned in close vicinity of the associated power generation facilities and the second converter station is positioned in a remote facility, such as a land-based facility. Such first and second converter stations are typically electrically connected via submerged high voltage direct current (HVDC) electric power cables that at least partially define an HVDC transmission system.
Many of these known converter stations define an HVDC converter system and the converter stations include some combination of line commutated converters (LCCs) and capacitor commutated converters (CCCs). These converter stations typically use a control scheme that includes some combination of current control (CC) loops, voltage control (VC) loops, and voltage dependent current order limits (VDCOLs). For example, the control scheme for the first converter station may include a CC loop and a VDCOL and the control scheme for the second converter station may include a margin control scheme, i.e., a combination of a CC loop with a VDCOL and a parallel VC loop, where the larger of the voltage commands and the current commands are used. Since the values of the commands are variable, the control scheme for the second converter station my shift modes between the CC loop and the VC loop on a frequent basis, thereby increasing a probability of system instability. Also, the use of VDCOLs may increase the frequency of the control loops hitting a limit without any further bandwidth to facilitate control, thereby increasing the probability of control overshoots and undershoots.
Also, many known HVDC converter systems include a large number of capacitor banks, AC harmonic filters, and DC-side harmonic filters (for filtering out DC-ripple) installed in the associated AC switchyards to compensate for harmonic currents and reactive power. In addition, such capacitor banks require associated electrical switchgear for placing the banks in service and removing them from service. Such capacitor banks, associated switchgear, and AC and DC harmonic filters are capital-intensive due to the land required and the amount of large equipment installed. In addition, a significant investment in replacement parts and preventative and corrective maintenance activities increases operational costs.
In addition, many known control systems for converter stations facilitate commutation margin angles, i.e., extinction angles to fall below predetermined thresholds. Therefore, increased reactive power flow, commutation failure, and lower margins to recovery from DC system disturbances may become more probable.